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In the diagram at 4 minutes 30 seconds, the labels for h_1 and h_2 are mixed up. Sorry about that!
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Everything I am talking about here is standard material of undergrad physics textbooks. My personal favorite is good old Goldstein https://en.wikipedia.org/wiki/Classic...)
On variational principles more specifically I quite like this textbook: https://link.springer.com/book/10.100...
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0:00 Intro
0:45 Optimization
1:35 Shortest Path
3:20 Least Time
5:36 Least Action
10:27 Quantum Mechanics
11:53 Sponsor Message